Microwave FETs including MESFETs, HEMFETs and the like used in a microwave circuit have a need to have a short gate length in order to suppress noise. However, should the gate length be shorter, the gate resistance would become higher, and accordingly, it is very difficult to suppress noise in a microwave FET. Accordingly, a gate electrode having a T-like cross-sectional shape, which can provide not only with shortened gate length, but also a reduced gate resistance has been proposed. For example, TECHNICAL REPORT ED-90-86-92 published by the Association of Electronic Data Communication, pages 37 to 43, discloses a conventional method of forming T-shaped gate electrode.
FIGS. 6A to 6D are sectional views for explaining the process steps for forming a conventional T-shape electrode, disclosed in the above-mentioned document.
In the conventional forming method, at first, as shown in FIG. 6A, a lower resist layer 12 having a low sensitivity and a high resolution, and an upper resist layer 14 having a high sensitivity are successively formed on a GaAs substrate 10, successively.
Then, as shown in FIG. 6B, the upper and lower resist layers 14, 12 are exposed by an electron beam type direct plotting apparatus, and thereafter, they are developed to form an aperture 16 for exposing a surface of the GaAs substrate 10 through the resist layers 14, 12. The upper resist layer 14 is dissolved by a developer to a greater extent in comparison with lower resist layer 12 even though both upper and lower resist layers 14, 12 are exposed at an equal exposure value, and accordingly, the aperture 16 having a T-like cross-sectional shape can be formed.
Next, as shown in FIG. 6C, a material layer 18 for forming a gate electrode is deposited on the surface of the substrate 10 and the lower resist layer 12 which are exposed through the aperture 16.
Further, as shown in FIG. 6D, the upper and lower resist layers 12, 14 are removed to obtain a gate electrode 20 having a T-like cross-sectional shape. The gate length can be effectively shorten by narrowing the width of the base part of the gate electrode 20, and further, the resistance of the gate electrode 20 can be reduced by widening the width of the head part of the gate electrode.
However, since the electron beam type direct plotting apparatus scans a resist layer with an electron beam to depict the shape of a gate electrode for exposure of the resist layer, a long time is consumed for depicting the gate electrode, causing a lowering of productivity.